Freestyle is an old project. It only ended up in 2.67 but was a gsoc during 2.4x series.
Custom Normals are more recent. Their support started in 2.74 and last feature addition about them was gsoc that ended up in 2.80.
So, Freestyle Face Smoothness option is not related to Custom Normals.
It is just related to the ability to smooth shading of only a selection of faces in edit mode.
It just means that Crease Angle value will be used for faces with Flat Shading and ignored for faces with Smooth Shading.
Wireframe shows a mesh with lots of beveled edges.
Crease Angle setting will not help in that case. Because on each side of bevel, edge loop is electable to become a freestyle line.
Sincerely, wireframe itself has difficulties to distinguish those lines. So Freestyle has no chance to do it without help.
Initial way to determine such lines is by using Suggestive Contours and Ridges & Valleys Edge Types in Line Set.
Those automatically contours were rarely satisfying by default.
That means that you have to play with Sphere Radius and Kr Derivative Epsilon values (only accessible when Advanced Options checkbox is ticked).
That is also the reason why Splitting options are numerous. To split lines obtained from those options.
That was not intuitive at all. So, other options were added.
Ability to take into account, material boundaries and edge marks.
You can make a selection of edge loops in edit mode, mark them as Freestyle edges, tick only Edge Mark option In Line Set panel. And those edges are the only one that Freestyle renders.
View Map cache option is there to avoid to redo the computation at each frame of an animation.
If you don’t make an animation, let option disabled by default.
At period, it was added, people were making scenes with less geometry.
If you were using Bevel shading node and textures for detail, that caching would be less longer.
Freestyle was created when Blender Internal was the only official render. People were used to wait 30, 45 or 60 minutes to render one frame.
It is clearly not comparable to nowadays rendertimes, with denoisers and EEVEE.
That is why LANPR is supposed to replace Freestyle in a near future.